1 // RUN: %clang_cc1 -verify -std=c++11 %s
2 // expected-no-diagnostics
3
4 // A direct proof that constexpr is Turing-complete, once DR1454 is implemented.
5
6 const unsigned halt = (unsigned)-1;
7
8 enum Dir { L, R };
9 struct Action {
10 bool tape;
11 Dir dir;
12 unsigned next;
13 };
14 using State = Action[2];
15
16 // An infinite tape!
17 struct Tape {
TapeTape18 constexpr Tape() : l(0), val(false), r(0) {}
TapeTape19 constexpr Tape(const Tape &old, bool write) :
20 l(old.l), val(write), r(old.r) {}
TapeTape21 constexpr Tape(const Tape &old, Dir dir) :
22 l(dir == L ? old.l ? old.l->l : 0 : &old),
23 val(dir == L ? old.l ? old.l->val : false
24 : old.r ? old.r->val : false),
25 r(dir == R ? old.r ? old.r->r : 0 : &old) {}
26 const Tape *l;
27 bool val;
28 const Tape *r;
29 };
update(const Tape & old,bool write)30 constexpr Tape update(const Tape &old, bool write) { return Tape(old, write); }
move(const Tape & old,Dir dir)31 constexpr Tape move(const Tape &old, Dir dir) { return Tape(old, dir); }
32
33 // Run turing machine 'tm' on tape 'tape' from state 'state'. Return number of
34 // steps taken until halt.
run(const State * tm,const Tape & tape,unsigned state)35 constexpr unsigned run(const State *tm, const Tape &tape, unsigned state) {
36 return state == halt ? 0 :
37 run(tm, move(update(tape, tm[state][tape.val].tape),
38 tm[state][tape.val].dir),
39 tm[state][tape.val].next) + 1;
40 }
41
42 // 3-state busy beaver. S(bb3) = 21.
43 constexpr State bb3[] = {
44 { { true, R, 1 }, { true, R, halt } },
45 { { true, L, 1 }, { false, R, 2 } },
46 { { true, L, 2 }, { true, L, 0 } }
47 };
48 static_assert(run(bb3, Tape(), 0) == 21, "");
49
50 // 4-state busy beaver. S(bb4) = 107.
51 constexpr State bb4[] = {
52 { { true, R, 1 }, { true, L, 1 } },
53 { { true, L, 0 }, { false, L, 2 } },
54 { { true, R, halt }, { true, L, 3 } },
55 { { true, R, 3 }, { false, R, 0 } } };
56 static_assert(run(bb4, Tape(), 0) == 107, "");
57